The present invention relates to a polymer concrete composition, to methods for lining pipes and for filling girthweld concrete cutbacks using the composition and to a reusable mold for filling girthweld concrete cutbacks.
In the laying of pipes or other conduits on the ocean floor, individual sections or lengths of the pipe or conduit, often being of a considerable length (e.g., 12 meters), are conventionally coated with concrete to give them a "negative buoyancy" so that the pipe will remain on the ocean floor during use. This is particularly necessary when a pipe having a diameter of greater than about 50 centimeters is used in transporting gas.
In general, the concrete coating is applied on shore and the individual coated pipe sections are then loaded on a barge for subsequent welding and installation. To allow the welding of the individual sections to one another, a small portion of each end of the pipe remains uncoated. Therefore, when welded, the joined pipe sections have an uncoated gap. To enable the pipe to enter the sea smoothly and to pro vide the joined pipe section with protection against physical and chemical damage once on the ocean floor, it is necessary to fill this girthweld concrete cut back.
There are various properties which any filler composition posses to be effectively employed in filling a girthweld concrete cutback. Specifically, to provide an efficient pipe laying operation, this filling operation including any required setting of the filler composition, is preferably completed in a relatively short time period of 10 to 15 minutes or less. In addition, moisture is commonly present during the cutback fill operation. Therefore, the filler composition must set and/or cure within this time period and in the presence of moisture to a material having sufficient physical properties such that the filled pipe can be further processed without damage.
The materials used to date in filling concrete cutbacks have not normally met these requirements. For example, the filling of girthweld concrete cutbacks has been accomplished by placing a metal sleeve or mold over the gap and filling this sleeve with molten mastic which solidifies as it cools. Unfortunately, the mastic does not set to a sufficiently strong material within the required time to allow further processing of the pipe without additional reinforcement such as the metal sleeve being present. Therefore, in conventional filling operations, the sleeve is not detached from the pipe (is not reusable) and sinks with the pipe to the ocean floor. This causes various other problems such as destroying fishnets by snagging. Moreover, filling cutbacks using the described method is labor intensive.
It has also been proposed to use a rigid polyurethane for filling girthweld concrete cutbacks. Unfortunately, the polyurethane materials generally exhibit an insufficient compressive strength to be employed without a metal sleeve or other reinforcement in the pipe-laying operation. In addition, the polyurethanes are, in general, absorbent which may cause more rapid corrosion of the pipe when it is exposed to the sea water.
Due to the action of the environment, over a period of time cement and/or concrete pipes, particularly sewer pipes, corrode. In the case of sewer pipes, for example, the H.sub.2 S present in the environment contained by the walls of the pipe reacts with oxygen to form sulfuric acid. The acid reacts with the calcium carbonate contained in the pipe to form calcium sulfate and carbon dioxide. Calcium sulfate is a soft material which is susceptible to crumbling and is easily washed away. Thereby causing a reduction in the wall thickness of the pipe. The carbon dioxide causes surface embrittlement of the pipe.
As a means for reducing the corrosion of cement or concrete pipes, it has been proposed to line the pipe with a chemically resistant material. Although the pipe is cleaned and dried to some extent, often there is residual amounts of water present in the pipe or on the pipe's surface. Therefore, the composition selected to coat the pipe is required to be sufficiently resistant to this moisture such that it is capable of forming a continuous coating which adheres to the interior surface of the pipe. The coating must also be sufficiently chemically resistant to protect the pipe from corrosive environments in the pipe. These requirements have severely limited the choice of materials which can be employed to coat the interior of the pipe.
For example, it has been proposed to reduce corrosion of a concrete pipe by lining both new and used (previously installed) concrete pipes with polyvinylchloride (PVC). Even with a PVC liner in a new concrete pipe, the joints between two individual pipes are not protected and are still susceptible to corrosion. In coating used or previously installed concrete pipes with PVC, it has been proposed to use an in situ process whereby an endless tube of PVC is placed into the pipe and hot water or other means used to push the PVC against the pipe walls and to adhere the PVC liner to the concrete. This method for lining pipes is very expensive and labor intensive.
Another proposed method is to use a glass reinforced plastic, such as glass reinforced epoxy resin, to coat the interior walls of a concrete pipe. Glass reinforced plastic materials are difficult to apply to the interior surface of the concrete pipe. In order to effectively reduce corrosion, these materials must be applied as a relatively thick coating which reduces the inside diameter of the pipe. This is an undesirable effect.
In view of the deficiencies of the prior art, it remains highly desirable to provide a composition which possesses the required properties such that it can be effectively employed in the presence of water often found in filling girthweld concrete cutbacks or in a cleaned but not necessarily completely dried sewer pipe.